1. Field of the Invention
The present invention relates to a hard carbon thin film and a method of forming the hard carbon thin film.
2. Description of the Related Art
Hard carbon thin films exhibit excellent hardness, resistivity, chemical stability and the others, and have gathered expectations for their applications to functional thin films for electronic devices and semiconductors, e.g. protective coatings on sliding parts for compressors such as rotary compressors, protective coatings on blades such as electric shaver blades, protective coatings on masks for screen deposition as well as on squeegees, constituent film layers of solar cells, protective coatings on thin film magnetic heads, and protective coatings on SAW devices.
In the above applications, poor adhesion of the hard carbon thin film to an underlying layer becomes problematic occasionally. A technique to improve its adhesion to the underlying layer such as a substrate has been proposed which provides a silicon interlayer between the underlying layer and the hard carbon thin film (See, for example, Japanese Patent Laying-Open No. Hei 1-317197(1989)).
Although the conventional techniques such as mentioned above have a potential advantage of imparting increased adhesion, delamination of the hard carbon thin film from the underlying layer is disadvantageously occasioned when influenced by the internal stress of the hard carbon thin film which becomes greater as a thickness thereof increases. Also, the interlayer must be formed in a separate step which results in a complicated fabrication.
In view of the above, there has been a continuing need for a hard carbon thin film which is capable of exhibiting an improved adherence to an underlying layer such as a substrate.
A crystalline hard carbon thin film, as generally called a diamond thin film, is typically formed through thermal decomposition of a material gas such as methane using a hot filament. Such a technique is however accompanied by the elevation of a substrate temperature up to about 1000xc2x0 C. which limits the material type of a substrate to be used. Also, the diamond thin film thus formed generally exhibits a large surface irregularity, which necessitates post-polishing thereof to smooth the surface, such as for use as a surface acoustic wave device.
A diamond-like thin film mainly consisting of non-crystalline or amorphous components has also been known as illustrative of the hard carbon thin film. Such a diamond-like thin film is generally formed using a plasma CVD technique which permits the formation thereof at a reduced substrate temperature around a room temperature. The diamond-like thin film thus formed is superior in surface smoothness but is generally inferior in surface hardness to the diamond thin film.
Accordingly, there remains a need for a technique which is capable of forming diamond thin films having smaller surface irregularities at reduced substrate temperatures, and another need for a technique which is capable of forming diamond-like thin films having increased surface hardnesses. Such needs would be met if a technique is provided which can control to some extent those mechanical properties of the diamond and diamond-like thin films to form hard carbon thin films with tailored properties. However, such a technique has not been reported up to date.
It is an object of the present invention to provide a hard carbon thin film which exhibits a satisfactory hardness as well as a good adhesion to an underlying layer such as a substrate, and a method of forming the hard carbon thin film.
It is another object of the present invention to provide a method for forming a hard carbon thin film which can control a proportion of crystalline and non-crystalline components in the thin film as well as its film properties such as hardness and surface smoothness.
A hard carbon thin film in accordance with a first aspect of the present invention characteristically has a graded structure in which a ratio of sp2 to sp3 carbon-carbon bonding (hereinafter referred to as xe2x80x9csp2/sp3 ratioxe2x80x9d) in thethin film decreases in its thickness direction from a film/underlayer interface toward a surface of the thin film.
A hard carbon thin film in accordance with a second aspect of the present invention comprises two or more layers having respective individual sp2/sp3 ratios different from each other, so that the sp2/sp3 ratio in the thin film decreases in a stepwise manner in its thickness direction from a film/underlayer interface toward a surface of the thin film.
A hard carbon thin film in accordance with a third aspect of the present invention characteristically has a graded structure in which the sp2/sp3 ratio in the thin film in its thickness direction decreases from a film/underlayer interface to a minimum and increases therefrom toward a surface of the thin film.
A hard carbon thin film in accordance with a fourth aspect of the present invention characteristically comprises three or more layers having respective individual sp2/sp3 ratios different from each other, so that the sp2/sp3 ratio in the thin film decreases in a stepwise manner in its thickness direction from a film/underlayer interface to a minimum and increases therefrom in a stepwise manner toward a surface of the thin film.
The sp2 and sp3 carbon-carbon bondings indicate different forms of chemical bonding between carbon atoms. It is generally known that the carbon-carbon bonding in the diamond thin film is predominantly sp3 while that in a graphite is predominantly sp2. It is also known that an amorphous diamond-like carbon thin film, as well as a partially crystalline diamond-like carbon thin film, may have a structure in which both sp2 and sp3 carbon- carbon bondings are distributed therethrough. In the present invention, such a sp2/sp3 ratio is characteristically varied in a film thickness direction as described earlier.
In the present invention, the sp2/sp3 ratio is varied preferably in the range of 0-3. Accordingly, the present invention is intended to include the case where the sp2/sp3 ratio is zero, i.e., the carbon-carbon bonding in the thin film is essentially devoid of sp2 and predominantly of sp3.
In general, the increased sp2/sp3 ratio, accordingly the increased proportion of sp2 carbon-carbon bonding tends to cause a decrease in internal stress to provide better adhesion to an underlying layer such as a substrate. On the other hand, the reduced sp2/sp3 ratio, accordingly the increased proportion of sp3 carbon-carbon bonding tends to produce a film with increased hardness and internal stress.
The sp2/sp3 ratio as specified in the present invention can be determined such as by an electron energy loss spectroscopy (EELS).
In the present invention, the hard carbon thin film is contemplated to include a crystalline diamond carbon thin film, an amorphous diamond-like carbon thin film, and a diamond-like carbon thin film having a partial crystalline structure. Accordingly, the change of sp2/sp3 ratio in a thickness direction of a thin film may be accompanied by the change in proportion of crystalline and non crystalline components in the thickness direction of the film.
The hard carbon thin film according to the present invention can be formed using generally-employed film-forming techniques. Foremost among those techniques are plasma CVD techniques including an ECR plasma CVD technique. A hot-filament CVD technique may also be used. Such techniques as to physically form thin films may also be applicable which include a sputtering technique and an ion beam deposition technique using an ion gun. Furthermore, the thin film may be formed using any combination of the above-mentioned plasma CVD, hot-filament CVD, sputtering and ion-beam deposition techniques.
The hard carbon thin film of the present invention may be formed on an underlying layer such as a substrate through an interlayer interposed therebetween. The material types of the interlayer include Si, Ti, Zr, W, Mo, Ru, Ge and oxides, nitrides and carbides thereof. The interlayers comprised of such materials can be formed such as by a magnetron RF sputtering technique. For example, any of those metallic elements can be sputtered in an argon plasma to form the interlayer. An oxygen or nitrogen gas may be introduced into a chamber during the sputtering to form the interlayer comprised of oxides or nitrides of any of those elements. The interlayer is formed to a typical thickness in the range of 20 xc3x85-300 xc3x85.
A first method for forming the hard carbon thin film of the present invention using a plasma CVD technique is characterized in that ion species, associated with formation of the thin film, in a plasma are varied in kinetic energy with film-forming time, so that the sp2/sp3 ratio in the hard carbon thin film is varied in its thickness direction. In order to vary the kinetic energies of those ion species, an acceleration voltage may be applied to them by applying a voltage to a grid disposed between a plasma generation chamber and a substrate, for example.
A second method for forming the hard carbon thin film of the present invention using a plasma CVD technique is characterized in that a varied amount of a hydrogen gas is admitted to a reaction system for its change with film-forming time, so that the sp2/sp3 ratio in the thin film is varied in its thickness.
A third method for forming the hard carbon thin film of the present invention using a plasma CVD technique is characterized in that a substrate temperature is varied with film-forming time, so that the sp2/sp3 ratio in the thin film formed on the substrate is varied in its thickness direction.
A fourth method for forming the hard carbon thin film of the present invention using a plasma CVD technique is characterized in that the proportion of ion species associated with formation of the thin film is varied with film-forming time, so that the sp2/sp3 ratio in the thin film is varied in its thickness direction. Those ion species associated with formation of the thin film include CH3+ and CH2, for example. The sp2/sp3 ratio in the thin film can be varied in its thickness direction by varying the proportion of those ion species with film-forming time.
The above-described first through fourth methods may be performed independently or in any combination thereof.
Furthermore, the hard carbon thin film of the present invention can be formed using a technique in accordance-with a fifth aspect of the present invention which will be described hereinafter.
The film-forming method of the present invention characteristically utilizes a plasma CVD process in varying the ion species associated with formation of the thin film to thereby vary the composition or structure of the thin film in its thickness direction. The composition or structural gradient in a thickness direct ion of the thin film can be produced such as by varying the sp2/sp3 ratio in a thickness direction of the thin film, e.g., by varying the ion species, such as CH3+ and CH2+ as described above as being associated with formation of the thin film, with film-forming time.
In accordance with a fifth aspect of the present invention, a method is provided for forming a hard carbon thin film through decomposition of a material gas. A characteristic feature of the method is that the material gas is decomposed using a technique of exposing it to heat and/or to a plasma whereby the film properties of the resulting hard carbon thin film can be controlled.
The method in accordance with the fifth aspect of the present invention combines a thermal decomposition technique, which is suited for formation of hard carbon thin films having higher degrees of crystallinity such as diamond thin films, and a plasma assisted decomposition technique which is suited for formation of hard carbon thin films having a major proportion of amorphous components such as diamond-like thin films, to thereby control a proportion of crystalline and non crystalline components in the hard carbon thin film and accomplish the control of its film properties such as hardness and surface smoothness.
Illustrative of the thermal decomposition technique is a technique which thermally decomposes the material gas by exposing it to heat from a hot filament disposed above a substrate on which the hard carbon thin film is to be deposited.
Exemplary techniques of forming the hard carbon thin films through plasma assisted decomposition of the material gas include generally-employed plasma CVD, radio-frequency (RF) plasma CVD, DC plasma CVD, and electron cyclotron resonance (ECR) plasma CVD techniques. The ECR plasma CVD technique is preferred when it is desired to form wide area hard carbon thin films.
In one embodiment practicing the method in accordance with the fifth aspect of the present invention, the film formation through the thermal decomposition of the material gas is followed by the additional film formation through the plasma assisted decomposition of the material gas. As discussed earlier, the use of thermal decomposition technique is effective in forming the hard carbon thin film having a higher degree of crystallinity. The succeeding film formation thereon using the plasma assisted decomposition technique is affected by the higher degree of crystallinity of the underlying hard carbon thin film to result in formation of the additional hard carbon thin film having an increased degree of crystallinity or hardness as a whole.
In another embodiment practicing the method in accordance with the fifth aspect of the present invention, the film formation through the thermal decomposition of the material gas is effected while the film formation through the plasma assisted decomposition of the material gas is in progress. Such a simultaneous occurrence of the thermal and plasma assisted decomposition of the material gas results in formation of the hard carbon thin film having a higher degree of crystallinity or hardness than when formed solely through the plasma assisted decomposition of the material
In the fifth aspect of the present invention, whether the hard carbon thin film formed has a crystalline diamond nature or an amorphous diamond-like nature depends upon the film-forming conditions respectively through the thermal decomposition and plasma assisted decomposition of the raw material gas. Accordingly, the suitable control of these film-forming conditions results in formation of thee hard carbon thin film having tailored film properties.
Also, whether the hard carbon thin film formed has a crystalline diamond nature or an amorphous diamond-like nature can be determined such as by a Raman spectroscopy, as will be described hereinafter.
In a further narrowed aspect of the present invention, a method for forming a hard carbon thin film comprises a first step and a subsequent second step. In the first step, a hard carbon thin film is formed using a first technique incorporating at least a film-forming technique through thermal decomposition of a material gas. The first step is followed by the second step in which an additional hard carbon thin film is formed thereon using a second technique incorporating at least a film-forming technique through decomposition of the material gas by a plasma, which is called both plasma decomposition and plasma assisted decomposition herein.
In the first step, a hard carbon thin film is formed by using the first technique incorporating at least the film-forming technique through thermal decomposition of a material gas. Accordingly, the hard carbon thin film may be formed by solely using the film-forming technique through thermal decomposition of the material gas. If desired, the first technique may further incorporate another film-forming technique, such as the film-forming technique through plasma assisted decomposition of the material gas, for simultaneous practice with the film-forming technique through thermal decomposition of the material gas.
In the second step, the additional hard carbon thin film is formed thereon using the second technique incorporating at least the film-forming technique through plasma assisted decomposition of the material gas. Accordingly, a hard carbon thin film may be formed by solely using the film-forming technique through plasma assisted decomposition of the material gas. If desired, the second technique may further incorporate another film-forming technique, such as the film-forming technique through thermal decomposition of the material gas, for simultaneous practice with the film-forming technique through plasma-assisted decomposition of the material gas.
Since in the first step, the hard carbon thin film is formed by using the first technique incorporating at least the film-forming technique through thermal decomposition of the material gas, a relatively high degree of crystallinity can-be imparted to the resulting hard carbon thin film. In the second step, the second technique is used to form the additional hard carbon thin film on the hard carbon thin film having the higher degree of crystallinity resulting from the first step, so that the relatively high degree of crystallinity of the underlying hard carbon thin film favorably affects the succeeding formation of the additional hard carbon thin film in the second step. Therefore, the hard carbon thin film can be formed which has a relatively high degree of crystallinity or hardness as a whole. Also, since the second technique incorporates at least the film-forming technique through plasma assisted decomposition of the material gas, amorphous components may be produced in the overlying hard carbon thin film formed by using the second technique, thereby imparting a relatively good surface smoothness, approaching at best to that of the diamond-like thin film, to the resulting hard carbon thin film.
In accordance with the present aspect, the second technique incorporating at least the film-forming technique through plasma assisted decomposition of the material gas at a relatively low temperature, when practiced subsequent to the first technique incorporating at least the film-forming technique through thermal decomposition of the material gas, imparts a smooth surface as well as an increased degree of crystallinity or hardness as a whole to the overlying hard carbon thin film.
In the fifth aspect of the present invention, in addition to admitting the material gas, a method further admits a hydrogen gas to a reaction system to thereby control film properties of the resulting hard carbon thin films. Introduction of the hydrogen gas contributes to removal of graphite components to permit selective formation of diamond thin films which have higher degrees of crystallinity and hardness.
Also in the fifth aspect of the present invention, a hard carbon thin film is formed on a substrate through an interlayer provided therebetween. The formation of the hard carbon thin film through the interlayer improves its characteristics, e.g. adhesion or adherence to the substrate. The interlayer may be comprised of a thin film of Si, Ti, Zr, Ge, or oxides or nitrides thereof. The film thickness of the interlayer is not particularly specified, but is preferably in the range of 20 xc3x85-1000 xc3x85.
In a sixth aspect of the present invention, a method is provided which forms an amorphous carbon coating on a substrate. Characteristically, a substrate is at its surface cleaned prior to formation of the hard carbon thin film thereon, and/or the hard carbon thin film is at its growth surface cleaned during formation thereof. The precleaning of the substrate serves to remove dusts, surface irregularities and scratches, which if present, provide undesirable growth surfaces on the substrate, to assure an uniform growth of the amorphous carbon coating on the substrate. Also, the cleaning or etching during coating formation serves to eliminate irregularity or unevenness of the coating growth surface to further insure the uniform growth of the amorphous carbon coating.
Ion or energy beam irradiation may be effected to clean the substrate surface prior to coating formation and/or the coating growth surface during the coating formation. In the ion beam irradiation, inert gas ions such as an Ar gas ion may be emitted such as by an ion gun. The condition of ion beam emission is not particularly specified, but generally at an ion current density of 0.01-5 mA/cm2, an acceleration voltage of 20-10,000 eV, and an inert gas partial pressure of 1xc3x9710xe2x88x925-1xc3x9710xe2x88x921 Torr.
An electron or laser beam may be employed to effect the energy beam irradiation. The electron beam may be emitted under a typical current density condition of 1xc3x9710xe2x88x922-1xc3x97101 A/cm2. The laser beam may be emitted under a typical power density condition of 1xc3x9710xe2x88x923-1xc3x97108 W/cm2. Laser beam sources include excimer, argon, YAG, CO2, Hexe2x80x94Cd, semiconductor, ruby lasers. Such an energy beam is generally scanned over the substrate surface or the coating growth surface. The energy beam may be provided in a pulsed form, if necessary.
Also in the sixth aspect of the present invention, in order for the substrate surface to be cleaned, the substrate may be irradiated with a plasma prior to coating formation thereon. The plasma may be an inert gas plasma, for example. A voltage may be applied to the substrate for acceleration of the produced plasma onto the substrate. For example, a radio-frequency voltage may be applied to the substrate so that a negative voltage is generated in the substrate which preferably has an absolute value of 20 V or higher.
In the case where the cleaning or etching is effected during coating formation, such a treatment preferably completes after the lapse of about one tenth of a total film-forming process from the start of coating formation.
For the method for forming an amorphous carbon film in accordance with the sixth aspect of the present invention, applicable film-forming techniques include general vapor phase epitaxial techniques, e.g., CVD techniques as represented by plasma CVD techniques such as the ECR plasma CVD technique and hot-filament CVD technique, sputtering and vacuum deposition techniques.
In accordance with the sixth aspect of the present invention, the surface cleaning treatment either prior to or during coating formation results in formation of an amorphous carbon coating which exhibits a surface roughness hrms not exceeding one fifth of a thickness thereof. Under a selected cleaning condition, an amorphous carbon coating may be formed which exhibit a surface roughness hrms not exceeding one tenth of a thickness thereof. The values of surface roughness hrms can be determined by using a stylus-based technique, and indicated by root-mean-square deviations from a mean surface.
An amorphous carbon coating in accordance with the sixth aspect of the present invention characteristically exhibits, immediately after formation thereof, a surface roughness hrms not exceeding one fifth, preferably one tenth of a thickness thereof. The amorphous carbon coating in accordance with the sixth aspect of the present invention exhibits such a surface roughness immediately after formation thereof, i.e., before any post-processing, such as polishing, is applied to a coating surface immediately after formation thereof.